Understanding Capacitors and Inductors: Essentials of Memory Elements in Circuits

1. 7/12/2010Lecture 8 Congratulations to Paul the Octopus on getting 8/8 world cup predictions correct Capacitors and Inductors

2. So far… • All circuits we’ve dealt with have reacted instantaneously • Change a resistance, voltage, or current, and everything else reacts instantly • Obviously this isn’t a complete model for electronic device. Why?

3. For the next 2 weeks • We’ll be talking about elements with memory • Capacitors • Inductors • Our first 6 lectures taught us how we can take a circuit schematic containing memoryless elements and convert them into algebraic equations • In the next 2 lectures we’ll talk about how to convert circuits with memory into differential equations • The next 3 after that will be about how we can use algebraic equations for circuits with memory if we have AC sources

4. Announcements • Midterm #2 will be on the 28th • Elements with memory • Make sure you do pre-lab before lab tomorrow • Who has finished it?

5. The Capacitor • The basic idea is pretty simple • Imagine you have two parallel metal plates, both of which have equal and opposite excess charges • Plates are separated by an insulating layer (air, glass, wood, etc) • The charges would love to balance out • Insulator blocks them (just as the ground blocks you from falling into the center of the earth)

6. The Capacitor • If you were to connect a resistive wire to the plates • Charges would flow through the wire • Charge flow is current • Energy is released as heat

7. The Capacitor • Remember that a voltage is the electrical potential between two points in space • Here, we have an imbalance of charge, and thus an electric field, and thus a voltage • Field strength is dependent on number and distribution of charges as well as material properties • Field length is dependent on size of capacitor • Capacitor size and material properties lumped into single “capacitance” C

8. The Capacitor • Thus, if you connect a voltage source to the plates • Like charges will move to get away from the source • Charge flow is current • Current will stop once charges reach equilibrium with voltage source, i.e. • Energy has been stored + - + - + -

9. The Capacitor Lots of current Zero current Zero VC VC=VS Lots of current [the other way] Zero current + - + - + - VC=VS Zero VC

10. iClicker Lots of current Zero current Acts like a: Short circuit Open circuit Resistor Voltage source Current source Zero VC VC=VS Lots of current Zero current + - + - High VC Zero VC

11. Extreme Corner Case • What happens if we short a charged capacitor? • Think of it is as the limit as resistance goes to zero: • Infinite current • Lasts only a very short time () until energy is released • Mathematically poorly behaved • Don’t do this

12. How much energy is stored? Strictly speaking we shouldn’t use t as our integration variable and also the limit that we’re integrating to, but you know what I mean…

13. Practical Capacitors • A capacitor can be constructed by interleaving the plates with two dielectric layers and rolling them up, to achieve a compact size. • To achieve a small volume, a very thin dielectric with a high dielectric constant is desirable. However, dielectric materials break down and become conductors when the electric field (units: V/cm) is too high. • Real capacitors have maximum voltage ratings • An engineering trade-off exists between compact size and high voltage rating

14. Capacitors • Useful for • Storing Energy • Filtering • Modeling unwanted capacitive effects, particularly delay

15. C Capacitor Symbol: Units: Farads (Coulombs/Volt) Current-Voltage relationship: C or C Electrolytic (polarized) capacitor These have high capacitance and cannot support voltage drops of the wrong polarity (typical range of values: 1 pF to 1 mF; for “supercapa- citors” up to a few F!) ic + vc – Note: vc must be a continuous function of time since the charge stored on each plate cannot change suddenly

16. Node Voltage with Capacitors ic + vc – • At the top right node, we write KCL • Current to the left through resistor: • Current down through the capacitor

17. Node Voltage with Capacitors ic + vc – • Or in other words • Or more compactly:

18. ODEs • Later today, we’ll talk about how to solve ODEs… • For now, let’s talk about inductors

19. Inductors • Capacitors are a piece of cake to understand, just rely on Coulomb’s Law • Inductors, by contrast, involve magnetic fields, and rely instead on Faraday’s Law • Comprehension comes with greater difficulty • Thus, we’ll treat inductors as mathematical objects and leave the derivation to Physics 7B (or page 467 of the book)

20. +  Two Fundamental Principles • The flow of current induces a magnetic field (Ampere’s Law) • A change in magnetic field through a loop of wire induces a voltage (Faraday’s Law) (Wikipedia)

21. +  Inductor Basics (1) • When we connect a voltage source to a wire, current clearly takes a little time to get moving • Thus, the magnetic field builds to some maximum strength over time

22. +  Inductor Basics (2) Current in a wire causes induces a voltage in any nearby circuit

23. Inductors Basics (3) • If we make a loop, the entire loop of wire will all contribute to the magnetic field through the loop • What’s more, this field will go through the loop producing the current! • Self induced voltage • Self inductance From: Dr. Richard F.W. Bader Professor of Chemistry / McMaster University

24. Inductors Basics (4) • More loops • More magnetic field generated • More circuit to receive magnetic field • Inductors are literally just loops of wire • Just like capacitors are just two conductors separated by an insulator (or a gap) • Just like resistors are just stuff with wires stuck to the ends University of Surrey http://personal.ee.surrey.ac.uk/Personal/H.M/UGLabs/components/inductors.htm

25. Inductors • Capacitors hold a voltage in the form of stored charge • Inductors hold a current in the form of stored magnetic field [dude…] • For webcast viewers, see drawings on board/notes for more comparison to capacitor

26. Symbol: Units: Henrys (Volts • second / Ampere) Current in terms of voltage: L (typical range of values: mH to 10 H) iL + vL – Note: iL must be a continuous function of time

27. Summary Capacitor v cannot change instantaneously i can change instantaneously Do not short-circuit a charged capacitor (-> infinite current!) In steady state (not time-varying), a capacitor behaves like an open circuit. Inductor i cannot change instantaneously v can change instantaneously Do not open-circuit an inductor with current (-> infinite voltage!) In steady state, an inductor behaves like a short circuit.

28. Ordinary Differential Equations • Inductors, too, give us a simple 1st order relationship between voltage and current • Node Voltage with memoryless circuits gave us algebraic equations • Node voltage with elements with memory will give us Ordinary Differential Equations (ODEs) • Next week will be a bunch of setting up and solving 1st and 2nd order linear ODEs • Higher order and especially nonlinear ODEs are tough to solve. For example…

29. Chua’s Circuit • ODEs are: – response of diodes and resistor block

30. Chua’s Circuit • Despite simplicity of ODEs • Exhibits chaos! Invented by current UC Berkeley EECS professor Leon Chua in 1983